Risk of transmission of SARS-CoV-2 on international flights, a retrospective cohort study using national surveillance data in England.

ABSTRACT

BACKGROUND: It is not yet fully understood to what extent in-flight transmission contributed to the spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). This study aimed to determine the occurrence and extent of SARS-CoV-2 transmission in-flight and assess factors associated with transmission risk to inform future control strategies. METHODS: Retrospective cohort study using data obtained from contact tracing of international flights arriving in England between 02/08/2021-15/10/2021. Transmission risk was estimated by calculating the secondary attack rate (SAR). Univariable and multivariable analyses of the SAR by specific risk factors was undertaken, including number of in-flight index cases; number of symptomatic index cases; contact vaccination status; flight duration; proximity to the index case(s); contact age. RESULTS: 11,307 index cases linked to 667,849 contacts with 5,289 secondary cases reported. In-flight SAR was 0.79% (95% CI 0.77-0.81). Increasing numbers of symptomatic cases (when > 4 index cases compared to one index case aOR 1.85; 95% CI 1.40-2.44) and seating proximity to an index case (seated within compared to outside of two rows OR 1.82; 95% CI 1.50-2.22) were associated with increased risk of secondary cases. Full vaccination history was protective (aOR 0.52; 95% CI 0.47-0.57). CONCLUSIONS: This study confirms that in-flight transmission of SARS-CoV-2 occurred. There are factors associated with increased risk of infection. Contact tracing identified exposed persons who subsequently developed infection. A targeted approach to contact tracing passengers with the highest exposure risk could be an effective use of limited public health resources.